In general, as the angular width (hereinafter, referred to as “back projection phase width”) of projection data used for reconstruction is wider, the amount of noise of a reconstructed image is reduced. On the contrary, as the back projection phase width is narrower, the amount of noise of the reconstructed image is increased.
Japanese Patent No. 4360817 discloses a method in which in order to promptly perform a reconstruction process while avoiding noise irregularities in a reconstructed image, an image is reconstructed using the same back projection phase width irrespective of an image position. Specifically, the smallest back projection phase width that can be used in the image is used. As described in Japanese Patent No. 4360817, in the case where a constant back projection phase width is used irrespective of an image position, the amount of noise is stabilized because the back projection phase width becomes a certain value without influence of a reconstruction slice position or an scanning FOV. Further, since the back projection phase width becomes narrower, the temporal resolution becomes higher. On the contrary, in the case where projection data to be used is restricted, the back projection phase width becomes narrower. Thus, the noise is disadvantageously increased as a whole. Further, in the case where the back projection phase width is set wider, an artifact is disadvantageously generated due to extrapolation.
To address such a problem, Japanese Patent No. 4612347 proposes a method in which a weight regulated on the basis of a weight dependent on a cone angle is calculated to be used for back projection, so that more projection data to be reconstructed can be used. In the method of Japanese Patent No. 4612347, more projection data can be used, and thus an image with less noise can be obtained. Further, using the weight dependent on a cone angle, extrapolation errors can be reduced.